Magneto-resistive device including soft synthetic ferrimagnet reference layer

ABSTRACT

A memory device includes a data layer having a magnetization that can be oriented in first and second directions; and a synthetic ferrimagnet reference layer. The data and reference layers have different coercivities.

BACKGROUND

[0001] The present invention relates to magneto-resistive devices. Thepresent invention also relates to data storage.

[0002] Magnetic Random Access Memory (“MRAM”) is a non-volatile memorythat is being considered for short-term and long-term data storage. MRAMhas lower power consumption than short-term memory such as DRAM, SRAMand Flash memory. MRAM can perform read and write operations much faster(by orders of magnitude) than conventional long-term storage devicessuch as hard drives. In addition, MRAM is more compact and consumes lesspower than hard drives. MRAM is also being considered for embeddedapplications such as extremely fast processors and network appliances.

[0003] A typical MRAM device includes an array of memory cells, wordlines extending along rows of the memory cells, and bit lines extendingalong columns of the memory cells. Each memory cell is located at across point of a word line and a bit line.

[0004] The memory cells may be based on tunneling magneto-resistive(TMR) devices such as spin dependent tunneling (SDT) junctions. Atypical SDT junction includes a pinned layer, a sense layer and aninsulating tunnel barrier sandwiched between the pinned and senselayers. The pinned layer has a magnetization orientation that is fixedso as not to rotate in the presence of an applied magnetic field in arange of interest. The sense layer has a magnetization that can beoriented in first and second directions: the same direction as thepinned layer magnetization or the opposite direction of the pinned layermagnetization. If the magnetizations of the pinned and sense layers arein the same direction, the orientation of the SDT junction is said to be“parallel.” If the magnetizations of the pinned and sense layers are inopposite directions, the orientation of the SDT junction is said to be“anti-parallel.” These two stable orientations, parallel andanti-parallel, may correspond to logic values of ‘0’ and ‘1.’

[0005] The magnetization orientation of the pinned layer may be fixed byan underlying antiferromagnetic (AF) pinning layer. The AF pinning layerprovides a large exchange field, which holds the magnetization of thepinned layer in one direction. Underlying the AF layer are usually firstand second seed layers. The first seed layer allows the second seedlayer to be grown with a (111) crystal structure orientation. The secondseed layer establishes a (111) crystal structure orientation for the AFpinning layer.

SUMMARY

[0006] A memory device according to the present invention includes adata layer having a magnetization that can be oriented in first andsecond directions; and a synthetic ferrimagnet reference layer. The dataand reference layers have different coercivities.

[0007] Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is an illustration of a magnetic memory device according tothe present invention.

[0009]FIG. 2 is an illustration of hysteresis loops for data andreference layers of the magnetic memory device shown in FIG. 1.

[0010]FIGS. 3a and 3 b are illustrations of a synthetic ferrimagnetreference layer for a magnetic memory device according to the presentinvention.

[0011]FIG. 4 is an illustration of a hysteresis loop for an individualferromagnetic layer of the synthetic ferrimagnet reference layer.

[0012]FIG. 5 is an illustration of a hysteresis loop for the syntheticferrimagnet reference layer.

[0013]FIG. 6 is an illustration of a first method of performing a readoperation on the magnetic memory device shown in FIG. 1.

[0014]FIGS. 7a and 7 b are illustrations of device magnetizationorientations corresponding to the first method.

[0015]FIG. 8 is an illustration of a second method of performing a readoperation on the memory device shown in FIG. 1.

[0016]FIGS. 9a-9 e and 10 a-10 e further illustrate the second method.

[0017]FIG. 11 is an illustration of a circuit for implementing thesecond method.

[0018]FIGS. 12a and 12 b are timing diagrams for the circuit shown inFIG. 11.

[0019]FIG. 13 is an illustration of an MRAM device according to thepresent invention.

[0020]FIG. 14 is an illustration of an alternative MRAM device accordingto the present invention.

[0021]FIG. 15 is an illustration of the alternative MRAM device during aread operation.

DETAILED DESCRIPTION

[0022] Referring to FIG. 1, a magnetic memory device 10 includes amagnetic tunnel junction 11 having a data layer 12, a reference layer14, and an insulating tunnel barrier 16 between the data and referencelayers 12 and 14. The data layer 12 has a magnetization (represented bythe vector M1) that can be oriented in first and second directions,typically along the easy axis (EA1) of the data layer 12. The referencelayer 14 has a magnetization (represented by the vector M2) that can beoriented in first and second directions, typically along its easy axis(EA2). The easy axes (EA1, EA2) are shown as extending along the x-axis.

[0023] If the magnetizations vectors (M1 and M2) of the data andreference layers 12 and 14 are pointing in the same direction, theorientation of the magnetic tunnel junction 11 is said to be “parallel.”If the magnetization vectors (M1 and M2) of the data and referencelayers 12 and 14 are pointing in opposite directions, the orientation ofthe magnetic tunnel junction 11 is said to be “anti-parallel. ” Thesetwo stable orientations, parallel and anti-parallel, may correspond tologic values of ‘0’ and ‘1.’

[0024] The insulating tunnel barrier 16 allows quantum mechanicaltunneling to occur between the data and reference layers 12 and 14. Thistunneling phenomenon is electron spin dependent, causing the resistanceof the magnetic tunnel junction 11 to be a function of the relativeorientations of the magnetization vectors (M1 and M2) of the data andreference layers 12 and 14. For instance, resistance of the magnetictunnel junction 11 is a first value (R) if the magnetization orientationof the magnetic tunnel junction 11 is parallel and a second value (R+ΔR)if the magnetization orientation is anti-parallel. The insulating tunnelbarrier 16 may be made of aluminum oxide (Al₂O₃), silicon dioxide(SiO₂), tantalum oxide (Ta₂O₅), silicon nitride (SiN₄), aluminum nitride(AlNx), or magnesium oxide (MgO). Other dielectrics and certainsemiconductor materials may be used for the insulating tunnel barrier16. Thickness of the insulating tunnel barrier 16 may range from about0.5 nanometers to about three nanometers.

[0025] Coercivity (H_(c1)) of the data layer 12 is much higher thancoercivity (H_(C2)) of the reference layer 14 (see FIG. 2, which showshysteresis loops L1 and L2 for the data and reference layers 12 and 14,respectively). The coercivity (H_(c1)) of the data layer 12 may be atleast 2-5 times greater than the coercivity of the reference layer 14.For example, the coercivity (H_(c1)) of the data layer 12 may be about25 Oe, and the coercivity (H_(c2)) of the reference layer 14 may beabout 5 Oe. Thus, the reference layer 14 is considered “softer” than thedata layer 12 because its magnetization vector (M2) is much easier toflip.

[0026] The data layer 12 is made of a ferromagnetic material. Thereference layer 14 is implemented as a synthetic ferrimagnet (SF), alsoknown as an artificial antiferromagnet.

[0027] Referring to FIGS. 3a and 3 b, the SF reference layer 14 mayinclude first and second ferromagnetic layers 50 and 52 separated by ametallic spacer layer 54. The ferromagnetic layers 50 and 52 may be madeof a material such as CoFe, NiFe or Co, and the spacer layer 54 may bemade of an electrically conductive, magnetically non-conductive materialsuch as Ru, Re, Rh or Cu. There is a strong interlayer exchange couplingbetween the two ferromagnetic layers 50 and 52. The magnitude of thiscoupling and also its sign (whether it is positive or negative) is afunction of the spacer thickness/material and ferromagnetic layermaterials and thicknesses. The coupling is negative, i.e., themagnetization vectors of the two ferromagnetic layers 50 and 52 areanti-parallel.

[0028] The size of the bit, its shape and the thickness of a particularFM layer decide its coercivity, i.e., the x-axis component of thehysteresis loop. One of the hysteresis loops is shown in FIG. 4. Thetotal volume of the FM layer and the unit magnetization (the magneticmoment per unit volume) of the layer material determine the totalmagnetization or moment of the layer, i.e., the y-axis component of thehysteresis loop.

[0029] The coercivities of the two FM layers 50 and 52 may be slightlydifferent (e.g., 10±5 Oe, 50±10 Oe). The coercivity of the SF referencelayer 14 is lower than that of the individual FM layers 50 and 52. Sincethe magnetizations of the two FM layers 50 and 52 point in oppositedirections, their moments tend to cancel each other, i.e., M_(SF)=M₁−M₂,where M₁ is the magnetic moment of the first ferromagnetic layer 50, M₂is the magnetic moment of the second ferromagnetic layer 52, and M_(SF)is the resultant magnetic moment of the SF reference layer 14. Resultingis the hysteresis loop of FIG. 5.

[0030] Thickness of the spacer layer 54 may be between about 0.2 nm and2 nm. Each ferromagnetic layer 50 and 52 may have, for example, acoercivity of about 10-100 Oe and similar hysteresis loops. If, forexample, the thickness of the first layer 50 is three nanometers and thethickness of the second layer 52 is four nanometers, the resultingimbalance will result in the hysteresis loop of equivalent to a 1 nmthickness. The resulting coercivity can be controlled to less than 10 Oeby varying the ratio of thickness of the first and second layers 50 and52. This low coercivity allows the magnetization vector of the SFreference layer 14 to be switched easily between the orientations shownin FIGS. 3a and 3 b.

[0031] Exchange coupling between the magnetization vectors of the two FMlayers 50 and 52 is very strong. Consequently, a very large magneticfield (e.g., 4000 Oe) would be needed to decouple the magnetizationvectors of the ferromagnetic layers 50 and 52.

[0032] Exemplary SF reference layers 14 are as follows. Example 1Example 2 Example 3 Thickness (nm) Layer 50 CoFe NiFe Co 3 Spacer 54 RuRu Ru 0.75 Layer 52 CoFe NiFe Co 4

[0033] The SF reference layer is not limited to the three-layerstructure just described. The SF reference layer may include more thanthree layers. For example, the SF reference layer may include thefollowing five reference layers: FM1/Ru1/FM2/Ru2/FM3, all havingdifferent thicknesses.

[0034] The SF reference layer may be clad with a ferromagnetic materialsuch as NiFe. The cladding can reduce stray fields and reduce read/writecurrent requirements (by concentrating the magnetic fields generatedduring read and write operations).

[0035] Returning to FIG. 1, a first conductor 18 extending along thex-axis is in contact with the data layer 12. A second conductor 20extending along the y-axis is in contact with the reference layer 14.The first and second conductors 18 and 20 are shown as being orthogonal.Above the second conductor 20 is a third conductor 22, which alsoextends along the y-axis. An electrical insulator 24 (e.g., a layer ofdielectric material) separates the second and third conductors 20 and22. The conductors 18, 20 and 22 are made of an electrically conductivematerial such as aluminum, copper, gold or silver.

[0036] Data may be written to the magnetic tunnel junction 11 bysupplying write currents to the first and second conductors 18 and 20.The current supplied to the first conductor 18 creates a magnetic fieldabout the first conductor 18, and the current supplied to the secondconductor 20 creates a magnetic field about the second conductor 20. Thetwo magnetic fields, when combined, exceed the coercivity (H_(c1)) ofthe data layer 12 and, therefore, cause the magnetization vector (M1) ofthe data layer 12 to be set in a desired orientation (the orientationwill depend upon the directions of the currents supplied to the firstand second conductors 18 and 20). The magnetization will be set toeither the orientation that corresponds to a logic ‘1’ or theorientation that corresponds to a logic ‘0’. Because the coercivity(H_(C2)) of the reference layer 14 is less than that of the data layer12, the combined magnetic fields cause magnetization (M2) of thereference layer 14 to assume that same orientation as the magnetization(M1) as the data layer 12.

[0037] After write currents are removed from the conductors 18 and 20,the magnetization vector (M1) of the data layer 12 retains itsorientation. The magnetization vector (M2) of the reference layer 14 mayor may not retain its orientation. If the reference layer 14 is“ultra-soft,” it will lose its magnetization orientation when the writecurrents are removed from the first and second conductors 18 and 20.

[0038] The third conductor 22 may be used to assist with writeoperations. By supplying a current to the third conductor 22 duringwrite operations, the resulting magnetic field about the third conductor22 combines with the other two magnetic fields to help set themagnetization vector (M1) of the data layer 12 in the desiredorientation.

[0039]FIG. 6 illustrates a first method of reading the magnetic memorydevice 10. A current is supplied to the third conductor 22, and theresulting magnetic field causes the magnetization vector (M2) of thereference layer 14 to assume a specific orientation (block 110). Theresulting magnetic field does not affect the magnetization vector (M1)of the data layer 12. Since the coercivity (H_(c2)) of the referencelayer 14 is low, the magnitude of the third conductor current may below.

[0040] As the current is supplied to the third conductor 22, a voltageis applied across the magnetic tunnel junction 11 (block 110). The firstand second conductors 18 and 20 may be used to apply the voltage acrossthe magnetic tunnel junction 11. The voltage causes a sense current toflow through the magnetic tunnel junction 11.

[0041] The resistance of the magnetic tunnel junction 11 is measured bysensing the current flowing though the magnetic tunnel junction 11(block 112). The sensed current is inversely proportional to theresistance of the magnetic tunnel junction 11. Thus I_(s)=V/R orI_(s)=V/(R+ΔR), where V is the applied voltage, I_(s) is the sensedcurrent, R is the nominal resistance of the device 10, and ΔR is thechange in resistance caused by going from a parallel magnetizationorientation to an anti-parallel magnetization orientation

[0042] Reference is now made to FIGS. 7a and 7 b. Consider a magnetictunnel junction 11 having a nominal resistance (R) of 1 Mohm, and atunneling magneto-resistance of 30%. A read current (I_(R)) is depictedas flowing into the third conductor 22. The read current (I_(R)) causesthe magnetization vector (M2) of the reference layer 14 to point to theleft. If the measured resistance R=1 Mohm, the data layer 12 stores afirst logic value (FIG. 7a). If the measured resistance R=1.3 Mohm, thedata layer stores a second logic value (FIG. 7b). Thus, by setting themagnetization of the reference layer 14 to a known orientation andmeasuring the resistance of the device 10 (either R or R+ΔR), the logicvalue stored in the magnetic memory device 10 is determined.

[0043]FIG. 8 illustrates a second method of reading the magnetic memorydevice 10. A bipolar pulse is applied to the third conductor 22 (block210), and transition of junction resistance is examined (212). Thedirection of the transition (that is, going from high to low, or low tohigh) indicates the magnetization orientation of the data layer 12 and,therefore, the logic value stored in the magnetic memory device 10.

[0044]FIGS. 9a-9 e further illustrate the second method in connectionwith a data layer 12 that stores a logic ‘0.’ A bipolar pulse 250 isapplied to the third conductor 22 (FIG. 9a). The bipolar pulse 250 has apositive polarity 252 (corresponding to a logic ‘0’) followed by anegative polarity 254 (corresponding to a logic ‘1’). The positivepolarity 252 orients the magnetization of the reference layer 14 in thesame direction as that of the data layer 12 (FIG. 9b), whereby themagnetization orientation of the device 10 is parallel and itsresistance value is R_(p). Then the negative polarity 254 orients themagnetization vector (M2) of the reference layer 14 in the oppositedirection (FIG. 9c), whereby the magnetization orientation of the device10 is anti-parallel and its resistance value is R+ΔR or R_(ap). Thus theresistance of the device 10 transitions from low to high (FIG. 9d). Thelow-to-high transition indicates that a logic ‘0’ is stored in thememory device 10. The corresponding sense current (I_(s)) is shown inFIG. 9e.

[0045]FIGS. 10a-10 e illustrate the second method in connection with adata layer 12 that stores a logic ‘1.’ The same bipolar pulse 250 isapplied to the third conductor 22 (FIG. 10a). The magnetic memory devicetransitions from an anti-parallel magnetization orientation (FIG. 10b)to a parallel magnetization orientation (FIG. 10c), whereby theresistance of the magnetic memory device 10 transitions from high to low(FIG. 10d). Thus the high-to-low transition indicates that a logic ‘1’is stored in the magnetic memory device 10. The corresponding sensecurrent (I_(s)) is shown in FIG. 10e.

[0046] The bipolar read operation references to itself. Therefore, thisdynamic approach is insensitive to resistance variations acrossdifferent devices.

[0047] The bipolar pulse is not limited to a single positive polarityfollowed by a single negative polarity, nor is it limited to a positivepolarity that corresponds to a logic ‘0’ and a negative polarity thatcorresponds to a logic ‘1’. For example, a positive polarity could justas easily correspond to a logic ‘1’, a bipolar pulse could begin with anegative polarity and transition to a positive polarity, etc.

[0048] A simple sense amplifier 310 for detecting the resistancetransition is shown in FIG. 11. The sense current (I_(s)) flowingthrough the magnetic tunnel junction 11 is supplied to a sense amplifier312. First and second outputs of the sense amplifier 312 provide avoltage (V_(SENSE)) that is proportional to sense current magnitude. Thefirst output is supplied to a first input (IN+) of a comparator 316. Thesecond output of the sense amplifier 312 is supplied to a delay element314, which has a delay of several nanoseconds. An output of the delayelement 314 is supplied to a second input (IN−) of the comparator 316.The comparator 316 compares the sense voltage (V_(SENSE)) at the firstcomparator input (IN+) to the delayed sense voltage at the secondcomparator input (IN−). An output (VOUT) of the comparator 316 indicatesthe logic state stored in the magnetic memory device 10.

[0049]FIGS. 12a and 12 b are timing diagrams for the circuit of FIG. 11.FIG. 12a corresponds to FIGS. 9a-9 e, and FIG. 12b corresponds to FIGS.9a-9 e.

[0050] The magnetic memory device 10 has a simpler structure than aconventional SDT junction. The magnetic memory device 10 is simpler tofabricate than an SDT junction because seed layers and an AF pinninglayer are not needed. Annealing of the data layer to set the easy axismay still be performed, but it is done at lower temperatures and is lesscritical. In addition, the complexity of the deposition process issignificantly reduced. Another advantage is that the data layer 12 is ontop of a metal conductor, resulting in a more uniform data film and,therefore, better magnetic response and manufacturability (in terms ofgreater uniformity over a wafer).

[0051] Reference is now made to FIG. 13, which illustrates an MRAMdevice 410 including an array 12 of magnetic tunnel junctions 11. Themagnetic tunnel junctions 11 are arranged in rows and columns, with therows extending along an x-direction and the columns extending along ay-direction. Only a relatively small number of the magnetic tunneljunctions 11 is shown to simplify the illustration of the MRAM device410. In practice, arrays of any size may be used.

[0052] Traces functioning as word lines 18 extend along the x-directionin a plane on one side of the array 12. The word lines 18 are in contactwith the data layers 12 of the magnetic tunnel junctions 11. Tracesfunctioning as bit lines 20 extend along the y-direction in a plane onan adjacent side of the array 12. The bit lines 20 are in contact withthe reference layers 14 of the magnetic tunnel junctions 11. There maybe one word line 18 for each row of the array 12 and one bit line 20 foreach column of the array 12. Each magnetic memory tunnel junction 11 islocated at a cross point of a word line 18 and a bit line 20.

[0053] Traces functioning as read lines 22 also extend along they-direction. The read lines 22 are on top of, and insulated from, thebit lines 20. (In the alternative, the read lines 22 may be beneath thebit lines 20, on top of or beneath the word lines 18, along rows orcolumns, etc.) The read lines 22 are independent of the word and bitlines 18 and 20.

[0054] The MRAM device 410 also includes first and second row decoders414 a and 414 b, first and second column decoders 416 a and 416 b, and aread/write circuit 418. The read/write circuit 418 includes a senseamplifier 420, ground connections 422, a row current source 424, avoltage source 426, and a column current source 428.

[0055] During a write operation on a selected magnetic tunnel junction11, the first row decoder 414 a connects one end of a selected word line18 to the row current source 424, the second row decoder 414 b connectsan opposite end of the selected word line 18 to ground, the first columndecoder 416 a connects one end of a selected bit line 20 to ground, andthe second column decoder 416 b connects the opposite end of theselected bit line 20 to the column current source 428. As a result,write currents flow through the selected word and bit lines 18 and 20.The write currents create magnetic fields, which cause the magnetictunnel junction 11 to switch. The column decoders 416 a and 416 b mayalso cause a write current to flow through the read line 22 crossing theselected magnetic tunnel junction 11. This third write current createsan additional magnetic field that assists in switching the selectedmagnetic tunnel junction 11.

[0056] During a read operation on a selected magnetic tunnel junction11, the first row decoder 414 a connects the voltage source 426 to aselected word line 18, and the first column decoder 416 a connects aselected bit line 20 to a virtual ground input of the sense amplifier420. As a result, a sense current flows through the selected magnetictunnel junction 11 to the input of the sense amplifier 420. In themeantime, the first and second column decoders 416 a and 416 b causeeither a steady read current or a bipolar current pulse to flow throughthe read line 22 crossing the selected magnetic tunnel junction 11. If asteady read current is supplied to the selected read line 22, theresistance state of the selected magnetic tunnel junction 11 is sensedby the sense amplifier 420. If a bipolar pulse is supplied to theselected read line 22, the transition of the junction resistance isexamined by the sense amplifier 420 (a sense amplifier 420 for examiningthe transition of the junction resistance may have the sameconfiguration as the sense amplifier 312 shown in FIG. 11).

[0057] The magnetic tunnel junctions 11 are coupled together throughmany parallel paths. The resistance seen at one cross point equals theresistance of the magnetic tunnel junction 11 at that cross point inparallel with resistances of magnetic tunnel junctions 11 in the otherrows and columns. Thus the array 12 of magnetic tunnel junctions 11 maybe characterized as a cross point resistor network.

[0058] Because the magnetic tunnel junctions 11 are connected as a crosspoint resistor network, parasitic or sneak path currents can interferewith the read operations on selected magnetic tunnel junctions 11.Blocking devices such as diodes or transistors may be connected to themagnetic tunnel junctions 11. These blocking devices can block theparasitic currents.

[0059] In the alternative, the parasitic currents may be dealt with byusing an “equipotential” method disclosed in assignee's U.S. Pat. No.6,259,644. If configured to use the equipotential method, the read/writecircuit 418 may provide the same potential to the unselected bit lines20 as the selected bit line 20, or it may provide the same potential tothe unselected word lines 18 as the selected bit line 20.

[0060] Because the read lines 22 are electrically insulated from the bitlines 20, they do not add to the resistive cross coupling of themagnetic tunnel junctions 11. Therefore, an equal potential is notapplied to the read lines 22.

[0061]FIG. 13 shows an MRAM device 410 with three different types oftraces: word lines 18, bit lines 20, and read lines 22. However, thepresent invention is not so limited. For example, an MRAM deviceaccording to the present invention may have only two different types oftraces: word lines 18 and bit lines 20.

[0062] Reference is made to FIG. 14, which illustrates an MRAM device510 including word lines 18 and bit lines 20, but not read lines 22.Magnetic tunnel junctions 11 are located at cross points of word and bitlines 18 and 20.

[0063] Additional reference is made to FIG. 15, which illustrates a readoperation using only the word and bit lines 18 and 20. The first rowdecoder 514 a connects the voltage source 526 to a selected word line18, and the first column decoder 516 a connects one end of a selectedbit line 20 to a virtual ground input of the sense amplifier 520. As aresult, a sense current (I_(s)) flows through the selected magnetictunnel junction 11 to the sense amplifier 520. The second column decoder516 b connects the column current source 528 to the other end of theselected bit line 20. As a result, a read current (I_(R)) flows throughthe selected bit line 20 to the sense amplifier 520. The read current(I_(R)) sets the magnetization vector of the reference layer. The senseamplifier 520 senses the sum of sense and read currents (I_(S)+I_(R)).Since the magnitude of the read current (I_(R)) is known, the magnitudeof the sense current (I_(s)), and hence the resistance and logic statesof the magnetic tunnel junction 11, can be determined.

[0064] Although the present invention was described in connection with aTMR device, it is not so limited. The present invention may be appliedto other types of magneto-resistive devices that have similaroperational characteristics. For instance, the present invention may beapplied to giant magneto-resistive (GMR) devices. A GMR device has thesame basic configuration as a TMR device, except that data and referencelayers are separated by a conductive non-magnetic metallic layer insteadof an insulating tunnel barrier. Exemplary spacer layer metals includegold, silver and copper. The relative orientations of the data andreference magnetization vectors affect in-plane resistance of a GMRdevice.

[0065] The present invention is not limited to GMR and TMR devices. Forinstance, the present invention may be applied to top and bottom spinvalves.

[0066] Although several specific embodiments of the present inventionhave been described and illustrated, the present invention is notlimited to the specific forms or arrangements of parts so described andillustrated. Instead, the present invention is construed according tothe claims the follow.

1. A magnetic memory device comprising: a data layer having amagnetization that can be oriented in first and second directions; and asynthetic ferrimagnet reference layer, the data and reference layershaving different coercivities.
 2. The device of claim 1, wherein thedata layer has a higher coercivity than the reference layer.
 3. Thedevice of claim 1, wherein the reference layer includes first and secondferromagnetic layers separated by a spacer layer, the first and secondferromagnetic layers having different coercivities.
 4. The device ofclaim 3, wherein the spacer layer is electrically conductive andmagnetically non-conductive.
 5. The device of claim 3, wherein thecoercivity of the reference layer is determined by the ratio ofthickness of the first and second ferromagnetic layers.
 6. The device ofclaim 3, wherein magnetic moments of the first and second ferromagneticlayers substantially cancel out.
 7. The device of claim 1, furthercomprising a first conductor on the first layer, an electrical insulatoron the first conductor, and a second conductor on the insulator.
 8. Thedevice of claim 7, further comprising a third conductor in contact withthe second layer, the third conductor being orthogonal to the firstconductor.
 9. The device of claim 1, further comprising a firstconductor in contact with the data layer, and a second conductor incontact with the reference layer, the first and second conductors beingorthogonal.
 10. The device of claim 1, further comprising a spacer layerbetween the data and reference layers.
 11. The device of claim 10,wherein the spacer layer is an insulating tunnel barrier.
 12. The deviceof claim 1, wherein the reference layer is not pinned.
 13. A referencelayer for a magneto-resistive device, the reference layer comprising:first and second ferromagnetic layers having different coercivities; anda spacer layer between the first and second layers.
 14. The referencelayer of claim 13, wherein the spacer layer is electrically conductiveand magnetically non-conductive.
 15. The reference layer of claim 13,wherein coercivity of the reference layer is determined by the ratio ofthickness of the first and second ferromagnetic layers.
 16. Thereference layer of claim 13, wherein magnetic moments of the first andsecond ferromagnetic layers substantially cancel out.
 17. The referencelayer of claim 13, wherein the reference layer is magnetically soft. 18.An information storage device comprising an array of memory cells, eachmemory cell including a data layer and a soft ferrimagnet referencelayer, the data and reference layers having magnetizations that can beswitched between first and second directions during write operations,only the second layer being switchable between first and seconddirections during read operations.
 19. The device of claim 18, furthercomprising electrically conductive and magnetically non-conductivespacers layers separating the data and reference layers.
 20. The deviceof claim 18, wherein reference layer coercivity is determined by theratio of first ferromagnetic layer thickness to second ferromagneticlayer thickness.
 21. The device of claim 18, wherein magnetic moments ofthe data and reference layers of a ferrimagnet reference layersubstantially cancel out.
 22. The device of claim 18, wherein the memorycells include magnetic tunnel junctions.